Existing barn ventilation systems are often inadequate on calm days. The temperature and relative air humidity in the barn may therefore rise to an unacceptable level. The animals may even become distressed. Heat stress develops when the cow is unable to give off body heat to the environment. This may occur at only 23°C and 70% air humidity. At extreme heat levels the forage intake is reduced causing lower stimulation of the rumen and lower milk production. Health problems may also occur, such as the risk of rumen acidification.